Depth filter cartridges formed of a nonwoven mass of a melt blown polymer are well known and have achieved widespread use in fluid filtration applications. Typically, such melt blown filter cartridges are made by extruding a polymer through orifices associated with a melt blowing die to form fibers which are directed toward a rotating forming mandrel. During melt blowing, a flow of inert gas (e.g., air) acts on the molten fibers so as to attenuate the fibers to relatively fine diameter and to randomly distribute the attenuated fibers onto the forming mandrel. Over time, therefore, a mass of nonwoven, randomly intermingled solidified fibers builds up on the forming mandrel. Controlled axial movement of the built-up mass of melt blown fibers relative to the melt blowing die will therefore allow a cylindrical filter cartridge of indefinite length to be formed continuously.
During melt blowing, it is well known that one or more of the process conditions associated with the melt blowing die may be varied so as to responsively achieve varied attributes of the resulting melt blown fibers in terms of fiber diameter and/or density, and thereby pore size. For example, in U.S. Pat. Nos. 3,904,798 and 3,801,400 (the entire content of each being incorporated expressly hereinto by reference), there is disclosed a technique whereby the (i) temperature of the polymer, (ii) speed of polymer extrusion, (iii) speed of rotation of the forming mandrel, (iv) distance between the melt blowing die and forming mandrel, and/or (v) weight of an idler roll associated with the forming mandrel, may be changed so as to gradually change the density of a melt blown fiber layer relative to a previously melt blown adjacent layer.
U.S. Pat. Nos. 4,594,202 and 4,726,901 (the entire content of each being incorporated expressly hereinto by reference) disclose a technique whereby the fiber diameter of the melt blown fibers is controllably varied to achieve substantially constant voids volume for each level of fiber diameter variance throughout the radial dimension (depth) of the filter cartridge without substantial fiber-to-fiber bonding.
In use, a filter cartridge must be capable of withstanding a significant pressure drop across its radial thickness (which pressure drop increases with increasing entrapment of particulates removed from the filtered fluid). In order to permit the filter cartridge to withstand pressure drops within design limits without collapse of the filter medium, it has been conventional practice to include a separate perforated tubular core around which the filter medium is disposed. (Please see in this regard, U.S. Pat. Nos. 3,933,557, 4,032,688 and 4,112,159, the entire content of each being expressly incorporated hereinto by reference).
It has also been proposed in the art to form "coreless" depth filter cartridges which include a nonwoven mass of melt blown fibers. For example, U.S. Pat. No. 4,240,864 (the entire content of which is incorporated herein expressly by reference) discloses a technique whereby a coreless melt blown filter cartridge may be made by varying the pressure applied to the accumulating fibers on the forming mandrel so as to obtain a filter of varying media density. Thus, although the diameters of the fibers in the various layers is substantially constant, by varying the pressure on such accumulated fibers from one zone to the next, a filter cartridge may be made having an innermost zone of sufficiently high density to serve as an integral "core".
More recently, U.S. Pat. No. 5,340,479 (the entire content of which is incorporated expressly hereinto by reference) has disclosed a depth filter cartridge formed of melt blown fibers having support filaments at a central area of the filter with diameters sufficiently large to thermally bind into a structure which is strong enough to support the remainder of the filter structure without collapse. This zone of relatively large diameter fibers thereby serves as an integral "core" for the remaining surrounding relatively small diameter filtration fibers.
While the above-described filter cartridges possess certain beneficial attributes, some improvements are still needed. For example, it would especially be desirable if a melt blown, nonwoven filter cartridge could be provided whereby one or more of the annular filtration zones was provided with integral support structure. It would furthermore be desirable if such integral support structure was in the form of continuously produced melt blown support fibers which are integrally co-located (i.e., intermingled) with concurrently melt blown filtration fibers within one or more of the annular filtration zones so that the filtration characteristics and pressure drop limits could be selectively "engineered" to meet specific end-use applications. It is towards fulfilling such a need that the present invention is directed.
SUMMARY OF THE INVENTION
Broadly, the present invention is embodied in filtration media formed of a mass of nonwoven melt blown support and filtration fibers which are integrally co-located with one another. The support fibers have, on average, relatively larger diameters as compared to the filtration fibers which are integrally co-located therewith. In preferred embodiments, the filtration media is disposed within at least one annular zone of a filtration element, for example, a disposable cylindrical filter cartridge having an axially elongate central hollow passageway which is surrounded by the filtration media.
The cylindrical filter cartridge according to the present invention will thus preferably include at least one annular filtration zone surrounding a central axially elongate hollow passageway and will integrally be formed of the co-located relatively larger diameter support fibers and relatively smaller diameter filtration fibers. A depth filter cartridge in accordance with the present invention may therefore be formed having one or more additional filtration zones (which additional filtration zones may or may not respectively be provided with integrally co-located support fibers) in annular relationship to one another. As a result of the melt blown, nonwoven fiber structure in accordance with the present invention whereby at least one filtration zone is comprised of relatively larger diameter support fibers integrally co-located with relatively smaller diameter filtration fibers, a filter cartridge having a wide range of "engineered" physical properties (e.g., in terms of filtration, pressure drop tolerance and the like) can be provided.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.